Raúl was sufficiently in control of his emotions to pass the notoriously vigilant US passport control without a hitch, despite his dodgy documents. And then he saw his dad. "He was waiting for me and I shouted out." The 10-year-old's voice trails off as he recalls the outburst of delight that attracted attention and got them both deported. "They led my father away in handcuffs. I started to cry. Then they sent me back too."

More than 35,000 Mexicans minors seeking to cross the northern border, about half of them unaccompanied, were repatriated last year. That is about 15% of the total number of deported nationals, and five times those registered in 2003. About the same number probably succeed in joining the 6-million strong Mexican community living illegally all over America, many of whom are suffering from the economic downturn but fear going home would be even worse.

Uriél González, who runs one of four hostels for deported children set up by the YMCA at crossing points, says the explosion of child migration is an unplanned consequence of US efforts to seal the border that began in the 1990s and intensified after 9/11.

"Before, the parents could go and visit their children in Mexico, but now they can't risk not getting back in," González says. "They are trying to re-establish their families in the north." This usually involves entrusting children, sometimes even babies, to people smugglers - known here as coyotes. It is an expensive business that carries with it terrible risks - some children disappear, perhaps kidnapped into prostitution, or even dying in the attempt.

Raúl's parents had chosen what is generally considered the safest option for their son, probably costing them upwards of $3,000 (£1,500). A relative took him from his grandmother's care in southern Mexico to Tijuana, where he was handed over to people smugglers contracted beforehand. A couple of days later he was given the stolen visa and passport of a boy of similar age and features. They took him to the snaking queue of people heading for the frontier, accompanied by a 15-year-old girl also trying to rejoin her family, and told him to keep his nerve. Then they left.

"I want to be with my mum and dad," Raúl says, with a shy smile, as he watches Stuart Little videos while waiting to be collected by the relative in a special unit for deported children on the Mexican side of the border. "I have a baby sister too, but she was born on the other side, so she has papers." Older children are often considered tough enough to join the mass of migrants paying $2,000 or more to follow coyotes over the inhospitable deserts, mountains and rivers that are the least monitored points along the 2,000-mile border. Most of the hundreds of Mexicans who perish on the border every year die on these treks.

"It was frightening, really frightening," says 16-year-old Diego, who grew up in central Mexico. Along with 22 others, Diego had set off into the stark rocky mountains east of Tijuana carrying little more than a few cans of tuna and a bottle of water. They reached the border after nightfall and kept walking in complete darkness, nervous about rattlesnakes and poisonous spiders, but ordered by the coyote never to use torches for fear of the border patrol. Spotted anyway, the group ran. Diego was caught when he tripped and fell, brushing a spiny plant that left deep gashes in his face. Lounging dejectedly on a bunk bed in the YMCA hostel, he is not sure what to do next. "I don't want to do that again," he says, "but I really want to be with my old man."

Coyotes typically allow their charges as many attempts to cross the border as it takes them to succeed, or to give up. A child in tow only adds to the pressure. Diana returned from California to collect her 10-year-old son, who she hadn't seen for over a year. The pair had endured seven failed treks across the mountains by the time she talked to the Guardian.

"I pack onions for $8 an hour in California while in Mexico I can earn 40 pesos a day. Four dollars a day! What would you do?" she asks. "I want to change the life of my kid, but I'm thinking it might not be possible now."